了解迭代器块和Dispose方法

我在看乔恩·斯基特的 Copenhagen C# talk 视频和我最终得到了这个代码。
问题:
代码打印完成后会发生什么。我的意思是为什么 iterator.MoveNext() 弱点?

CODE:

 class IteratorBlocks
    {
        public static IEnumerable<string> GetStringsForever()
        {
            string current = "";
            char nextChar = 'a';
            try
            {
                while (true)
                {
                    current += nextChar;
                    nextChar++;

                    if (nextChar > 'z')
                    {
                        nextChar = 'a';
                    }

                    yield return current;
                }
            }
            finally
            {
                Console.WriteLine("Finished");
            }
        }
    }

    class Program
    {

        static void Main(string[] args)
        {
            IEnumerable<string> strings = IteratorBlocks.GetStringsForever();

            IEnumerator<string> iterator = strings.GetEnumerator();

            for (int i = 0; i < 10; i++)
            {
                iterator.MoveNext();
                Console.WriteLine(iterator.Current);
            }

            /* 
               I am not able to get what the code is doing beyond this line?
             */

            iterator.Dispose();

            for (int i = 0; i < 10; i++)
            {
                iterator.MoveNext();
                Console.WriteLine(iterator.Current);
            }

        }
    }

OUTPUT:

a
ab
abc
abcd
abcde
abcdef
abcdefg
abcdefgh
abcdefghi
abcdefghij
Finished
abcdefghij
abcdefghij
abcdefghij
abcdefghij
abcdefghij
abcdefghij
abcdefghij
abcdefghij
abcdefghij
abcdefghij